Waking up and not being able to smell the coffee could be a sign of COVID-19. The World Health Organization recently announced that a sudden loss of sense of smell (anosmia) should be added to the symptoms of COVID-19.
The UK government has just done that, stating: "From today, all individuals should self-isolate if they develop a new continuous cough or fever or anosmia."
The inclusion of sudden anosmia is important as it may be an early – and sometimes the only – symptom of COVID-19 infection. It is also a symptom, like fever, that we can detect ourselves without a laboratory test. The indications are that if we were all to respond to this cue by self-isolating, we could reduce the spread of the virus.
An early clue that a loss of smell might be related to COVID-19 came in early March 2020 from a Facebook post about an ear, nose and throat (ENT) doctor who suddenly lost his sense of smell. What followed was a gradual accumulation of evidence linking anosmia to COVID-19.
Membership of closed Facebook groups dedicated to those experiencing smell loss grew exponentially, Google searches for "anosmia" increased, and high-profile cases were reported in the news.
ENT doctors saw an increase in patients reporting anosmia and started to publish case studies based on the experience of their patients – many of them healthcare professionals.
One, a neurosurgeon, reported anosmia with no other symptoms. Two days later he tested positive for COVID-19.
The sudden onset of anosmia was described as a new finding that may distinguish COVID-19 from the common cold or flu. An early statement was issued by ENT-UK and the British Rhinology Society calling for anosmia to be recognised as a marker of the virus.
However, while the case studies and anecdotes are compelling, they lack the large datasets of a scientific study.
The first large dataset came from Iran. It was a home-based study of 10,000 people showing a correlation between anosmia and COVID-19. Results from the second large dataset were published by a team based at King's College London.
Their COVID Symptom Study app showed that 65 percent of those who tested positive for the disease also had a loss of smell and taste, and this symptom was by far the best predictor of COVID-19. And preliminary results from the first survey of the Global Consortium for Chemosensory Research (GCCR) showed that those reporting COVID-19 symptoms had an average drop of 80 percent in their ability to smell.
These surveys add weight to the anecdotal evidence, but they have limitations. The main one being that they are based on people self-reporting symptoms – that is, their ability to smell was not clinically assessed.
On the other hand, studies based in hospitals and ENT clinics provide valuable evidence where the COVID-19 status and case histories can be determined with more certainty.
The first came out of China, where researchers reported a modest 5 percent of patients had an impaired sense of smell.
Studies in France (417 patients), in Italy (202) and in the US (102), on patients all testing positive for COVID-19, found a loss of smell in 86 percent, 64 percent and 68 precent of cases, respectively.
Results from clinical studies appear at the rate of one every few days and the conclusions overwhelmingly support the claim that anosmia is associated with COVID-19. The most revealing of these was another study from Iran that measured sense of smell using a recognised scratch and sniff test.
In this case, 59 out of 60 COVID-19 patients had a decrease in their sense of smell. Three systematic reviews of the recent evidence have concluded that there is a strong link between COVID-19 and anosmia.
Early warning sign
It is important to understand the timings of smell loss in relation to other symptoms. Several studies report the onset of anosmia before other symptoms, or as the only symptom. Indeed, a US study found anosmia appearing as the first symptom in 27 percent of their responses.
A Chinese group showed you can still be infectious, even if anosmia is your only symptom, but this is an area when more data is required fast. The relationship between anosmia and infectiousness is still unclear, but it is certainly an early warning sign that other symptoms may follow.
Considering the combined strength of all the above, there is little doubt that sudden loss of smell is related to COVID-19.
But we still need a strict evidence-based approach involving objective smell testing and evidence of the mechanisms involved, some of which is emerging. In the meantime, we would all be well advised to treat a sudden loss of the sense of smell as an indication we should self-isolate.
More data is needed, and you can help. If you have experienced a sudden loss of smell or taste during the pandemic, fill in both the UK and global GCCR surveys. These simple online surveys take less than ten minutes each. Our focus is on understanding the mechanisms involved in both smell and taste disorders. 
Jane Parker, Associate Professor, Flavour Chemistry, University of Reading; Carl Philpott, Professor of Rhinology and Olfactology, University of East Anglia, and Tristram Wyatt, Senior Research Fellow, University of Oxford.
This article is republished from The Conversation under a Creative Commons license. Read the original article.